Photoconductive switches and switch packages typically consist of a wide bandgap photoconductive material (e.g. GaN, ZnO, diamond, AlN, SiC, GaAs, BN, etc.), a source for energetic photons (e.g. a laser), a method to couple the laser into the switch, and a method for high voltage to enter and leave the switch package such as via electrodes positioned on opposite sides of the substrate. Arranged as such, the photoconductive switch package may be characterized as a three terminal device similar to transistors; with one of the terminals being a laser input or the voltage input to the laser system. When the photoconductive switch material is illuminated such as by a laser, the laser photons change the conductivity of the photoconductive material and make it viable as an optically controlled switch, capable of operating in the linear mode.
Photoconductive switches arranged vertically (i.e. electrodes on opposite side of a photoconductive substrate) that are subjected to high voltage can fail due to enhanced electric fields appearing at the triple point (where the conductor, substrate, and non-conductor are in contact) when the substrate is rendered conducting by the application of incident radiation.
Present design methods focus on shaping the electrodes and surrounding dielectric potting material to relieve these enhancements when the switch is in the off state (the condition when it is holding the maximum voltage). For example, FIG. 1 shows a switch design having electrodes with curved edge profiles intended to reduce edge enhancements. However, designing against the field enhancements when the switch is in the off state is often insufficient to prevent breakdown as the electric field distribution inside the substrate is profoundly modified once the substrate becomes conducting. For example, FIG. 2 shows the equipotential plot of the switch design of FIG. 1 in the “off” state, and illustrates how edge enhancement is reduced. And FIG. 4 shows the vertical field plot corresponding to the equipotential plot of FIG. 1. However, when the switch is in the “on” state, as shown in FIGS. 3 and 5, the vertical field has a field singularity at the electrode edge where, as shown particularly in FIG. 3, the equipotential lines must bend vertically around the electrode edge since the vertical field vanishes due to current not being able to flow into the insulator. Thus, while the resulting switches hold off substantial voltages when charged, a very large electric field enhancement can occur around the electrode edge when the substrate becomes conducting and breakdown at much lower charge voltages when laser light is applied to activate the switches, even when such an enhancement is totally absent when the switch is off.
There is therefore a need for an improved high voltage photoconductive switch that takes into account both operating states (switch off and on) to ensure that field enhancements under all conditions are minimized.